SOUTH PORTLAND, Maine — WEX Inc.’s $532.5 million purchase of a North Dakota company would be its largest single acquisition, reflecting a focus that has widened beyond its core business of processing fuel for corporate fleets.

WEX’s revenue from processing travel payments through sites like Priceline, Orbitz and Expedia.com, handling payroll processing and health care payments has grown by nearly half since 2008. Last year, those new markets made up more than a quarter of the company’s total revenue, up from less than 7 percent in 2008.

“We’re trying to position ourselves as a company that tackles business-to-business payment needs and the first example was in fleets,” said Micky Thomas, the company’s treasurer and vice payment for investor relations, in a telephone interview. “The common thread is to take a complex payment situation and provide security and control around the transaction.”

So far, he said, the company’s travel business has been the fastest growing, but its latest move signals big interest in other markets, too.

The company expects its cash-and-credit purchase of North Dakota-based health care payment processor Evolution1 to boost its annual revenue by at least $80 million and increase its stake in the health care payment processing market, estimated at $1 billion annually, according to CEO Melissa Smith.

Smith said projections for that market to increase more than twofold in five years were the “grounding thesis” of the decision to buy Evolution1, which processes payments for about 90,000 employers covering 10 million people in the United States and Canada.

“There’s more and more of a move toward consumer-driven health plans,” she said, referencing insurance plans like health savings accounts and health reimbursement accounts that covered about 9.7 percent of insured people in 2013, up slightly from 2012, according to a survey by the Employee Benefit Research Institute.

Investors echoed confidence in the acquisition Tuesday, when shares rose 5.3 percent to $104.12, up from $98.80 per share on the market’s close Monday.

Analysts at credit rating agency Standard & Poor’s, however, flagged the transaction as a potential cause for lowering the company’s “BB” credit rating, noting that financing the deal with cash and a portion of its $700 million line of credit could raise the company’s debt-to-earnings ratio higher than 3-to-1 in the next year.

In listing the company’s rating and the latest deal as under review, S&P also noted WEX’s plan to purchase ExxonMobil’s European fuel card business, Esso Card, in a deal expected to close later this year.

“A potential downgrade would reflect our view that the company’s willingness to fund such a large acquisition with debt is aggressive,” the ratings agency’s notice stated. “On the other hand, from a business standpoint, we believe the acquisitions of Evolution1 and Esso Card would increase the company’s scale and diversity.”

Evolution1 and its 300 employees will remain in Fargo, North Dakota, through the deal, with WEX running it as a standalone business “for the foreseeable future,” Smith said in a telephone interview Tuesday.

That brings the company to a total employee count of about 1,700, with more than 600 in Maine. That’s more than double the staff it had in 2005, the year the company, then Wright Express, went public.

In an interview last week, before announcement of the latest purchase, Thomas said the benefit for Maine in the company’s international expansion of its fleet card systems and growth in other markets is primarily for employees seeking out management opportunities.

Smith said the company has an ongoing vetting process for potential acquisitions that it identifies each fall.

The company expects the purchase announced Monday, which is still subject to regulatory approval, will close in the third quarter of this year.